With the expansion of applications of fiber-reinforced composite materials in recent years, various physical properties have become required of fiber-reinforced composite materials. One of the required properties is improved heat resistance. For fiber-reinforced composite materials, heat-curable resins such as phenol resins, melamine resins, bismaleimide resins, unsaturated polyester resins, cyanate ester resins, and epoxy resins are used as matrix resins depending upon specific use of fiber-reinforced composite materials, and the heat resistance of the fiber-reinforced composite materials relies upon the heat resistance of these matrix resins.
In order to improve the heat resistance of the matrix resins, an epoxy resin obtained by glycidylating 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane (see, for example, Japanese Patent No. 3573530) and a composition comprising diaminophenyl sulfone, polyether sulfone, or polyether imide mixed into an epoxy resin composed mainly of an epoxy resin component having a triglycidylaminophenol skeleton (see, for example, Japanese Patent Application Laid-Open No. 297316/1987) have been proposed.
Cyanate ester resins are known as resins that, when cured, form cured resins which have a triazine ring and possess a high modulus of elasticity and a high level of heat resistance. Accordingly, cyanate ester resins are suitable for use as the heat-curable resins. Processes for producing fiber-reinforced composite materials using cyanate ester resins as a matrix have also been proposed (see, for example, Japanese Patent Application Laid-Open No. 12819/2003 and Japanese Patent Application Laid-Open No. 70115/2006).
Due to a recent tendency towards an increasing demand to these materials for a higher level of properties in fields of application thereof, a further improvement in physical properties of the matrix resins has been desired.